Coping with herbicide-resistant weeds, after decades of the easy, sure control afforded by glyphosate (Roundup), will require a multi-pronged approach, Mississippi growers were told at the 2011 Delta Ag Expo. As glyphosate-resistant pigweed (Palmer amaranth) spreads in the Mid-South, and resistant Italian ryegrass continues to be documented, producers will need to include alternate chemistries — many of which are decades-old active ingredients — in their weed control programs, a panel of specialists noted.

'A definite change in the way we farm'

Larson: “This is definitely going to change the way we farm. Crop rotation is one practice that can play a significant role in dealing with resistant weeds.

“A rotation including corn serves as the most effective tool to combat resistant Palmer amaranth and marestail. However, ryegrass is a significant issue for corn production.

“We have documented glyphosate-resistant ryegrass in 12 Delta counties and ALS-resistant ryegrass in 14 counties. This is a very serious issue in corn, and control requires some very aggressive, proactive methods. Ryegrass can be a significant competitor for corn and it needs to be controlled before you ever put the planter in the field. One ryegrass plant per 7 square feet can result in corn yield loss of as much as 27 percent, or 65 bushels per acre, as documented in a Mississippi State University corn verification trial last year.

“This is why we’re recommending a more aggressive strategy with burndown with products other than glyphosate and 2,4-D.”

[A field day highlighting management programs for glyphosate-resistant Italian ryegrass will be held March 10 at the Capps Center on the campus of the Delta Research and Extension Center at Stoneville. Registration will begin at 7:30 a.m. Plans are to have an actual field tour of research plots, but the program will be moved indoors in the event of inclement weather.]

Dodds: “Glyphosate-resistant pigweed is a major threat to cotton. I saw more in-season tillage and chopping crews last year than in the previous six years combined. We can manage this problem, but growers are going to have to be willing to adapt and to adjust their practices.”

Stephenson: “Georgia has good data that turning under pigweed seeds can help control the weed, but if you have to grow a crop on a bed, it can bring the seed right back up.

“We’ve had only one confirmed case of glyphosate-resistant pigweed in Louisiana, but we have glyphosate-resistant ryegrass in several areas. We also are concerned about resistant water hemp, and we have confirmed resistant johnsongrass, which took massive amounts of herbicide to kill.